1. Field of the Invention
The present invention relates to a driver circuit for a laser used as an electrical-to-optical transducer in optical communication systems.
2. Background Information
To modulate the laser, use is commonly made of a differential amplifier. Below the so-called threshold current, the laser acts like a light-emitting diode, and the optical output power in this current range is very low. Above the threshold current, lasing occurs, and the optical output power P.sub.opt increases linearly with increasing modulating current I.sub.mod. The rise of the function P.sub.opt =f(I.sub.mod) is dependent on the laser type and specimen used and varies with temperature and aging. To obtain the typical optical output powers specified in the data sheet, a modulating current of I.sub.mod =5 mA is required for a 0.8-.mu.m laser of high quantum efficiency and low temperature, for example; for 1.55-.mu.m lasers of moderate quantum efficiency, I.sub.mod must be 40 mA.
Prior art driver circuits for lasers employ a differential amplifier which is adapted to the properties of the respective laser and optimized for maximum current, cf. H. M. Rein, "Multi-Gigabit-Per-Second Silicon Bipolar IC's for Future Optical Fiber Transmission Systems", IEEE Journal of Solid-State Circuits, Vol. 23, No. 23, June 1988, pages 664 to 675, and K. Yamashita et al, "Master-Slice Monolithic Integration Design and Characteristics of LD/LED Transmitter for 100-400 Mbit/s Optical Transmission Systems", Journal of Lightwave Technology, Vol. LT-4, No. 3, March 1986, pages 353 to 359. In the solutions described, the optimization for maximum current was performed because the transit frequency of a silicon bipolar transistor depends on the emitter-current density, cf. H.-M. Rein, R. Ranft, "Integrierte Bipolarschaltungen", Springer-Verlag Berlin Heidelberg 1980, pages 100 et seq. The transit frequency decreases with decreasing emitter-current density, and the depletion-layer capacitances and collector-substrate capacitances of the transistor increase with increasing transistor areas. As a result, the optimum bandwidth of the differential amplifier is achieved only within a small current range which depends essentially on the emitter area and corresponds approximately to operation at maximum emitter-current density. If, in such driver circuits, the emitter-current density is reduced by a factor of 10, for example, the influence of the parasitic capacitances on the pulse response will increase so that the bandwidth will be greatly limited and the pulse shape will be substantially degraded. Therefore, such driver circuits are only suitable for lasers with constant properties if no disadvantages in the transient response are to result.